In recent years, a hybrid vehicle comprising an engine and a motor generator has been proposed for the purpose of saving a fuel for driving the engine, reducing noise resulting from a rotation of the engine, and reducing emission resulting from combustion of the fuel. In the hybrid vehicle, the engine and the motor are controlled on the basis of a running condition of the vehicle when the vehicle is running.
Specifically, the hybrid vehicle can be run by driving the engine in a revolution region where combustion efficiency thereof is preferable, and also by using the motor generator as a motor while halting the engine in a revolution region where the combustion efficiency of the engine is degraded. One example of a control system for a drive system thus having an engine and a motor generator is disclosed in Japanese Patent Laid-Open No. 2003-127681. According to the system disclosed in Japanese Patent Laid-Open No. 2003-127681, an output torque of an internal combustion engine is transmitted to a wheel via a planetary gear mechanism, a transmission, a propeller shaft, a differential device and a vehicle axle. The planetary gear mechanism comprises a sun gear, ring gear and a carrier, and a crankshaft of the engine is connected with the carrier. Also, a first motor/generator is provided and a rotor thereof is connected with the sun gear. The ring gear is connected with the transmission. On the other hand, a second motor/generator is provided and a rotor thereof is connected with the propeller shaft. Japanese Patent Laid-Open No. 2003-127681 describes that torque of the vehicle axle is generated by the internal combustion engine and the second motor/generator, and a transmission having a plurality of planetary gear mechanisms, a clutch and a brake is disclosed therein as an example of the transmission. A first gear position, a second gear position, a third gear position and a reverse position of the transmission are achieved by controlling an engagement and a releasing of the clutch and the brake.
However, according to the vehicle taught by Japanese Patent Laid-Open No. 2003-127681, driving force may be fluctuated by a change in the output torque of the transmission when carrying out a shifting operation. Therefore, the driver and passenger may feel uncomfortable feeling due to such fluctuation in the driving force.